text: In biochemistry the substrate is known as a receptor. A receptor is a protein molecule, embedded in either the plasma membrane or the cytoplasm of a cell, to which one or more specific kinds of signalling molecules may bind. A ligand may be a peptide or another small molecule, such as a neurotransmitter, a hormone, a pharmaceutical drug, or a toxin. The specificity of a receptor is determined by its spatial geometry and the way it binds to the ligand through non-covalent interactions, such as hydrogen bonding or Van der Waals forces.
  If a receptor can be isolated a synthetic drug can be developed either to stimulate the receptor, an agonist or to block it, an antagonist. The stomach ulcer drug cimetidine was developed as an H antagonist by chemically engineering the molecule for maximum specificity to an isolated tissue containing the receptor. The further use of quantitative structure-activity relationships (QSAR) led to the development of other agents such as ranitidine.
  It is important to note that "selectivity" when referring to a drug is relative and not absolute. For example, in a higher dose, a specific drug molecule may also bind to other receptors than those said to be "selective".